The present invention relates to the field of data storage devices, and in particular to a read/write device for optical data storage devices utilizing surface plasmon-enhanced optical transmission through subwavelength apertures in metal films, which offers very high throughput and resolution.
As discussed in detail in U.S. Pat. No. 5,973,316 to Ebbesen et al., U.S. Pat. No. 6,040,936 to Kim et al., U.S. Pat. No. 6,052,238 to Ebbesen et al., U.S. Pat. No. 6,236,033 to Ebbesen et al., and U.S. Pat. No. 6,285,020 to Kim et al. (each of these patents being incorporated herein by this reference), light transmission through one or more subwavelength-diameter apertures provided in a thin metal (i.e. conductive and opaque in the wavelength of interest) film can be greatly enhanced by arranging the apertures in a periodic array and/or by providing a periodic surface topography on the metal film (surface features such as dimples or protrusions) in conjunction with the aperture(s). This enhancement, which can be as large as a factor of 1,000, occurs when light incident on the conductive film interacts resonantly with a surface plasmon mode.
Optical storage disks, such as CD-ROM and DVD, are becoming increasingly attractive data storage media because of their high data densities, compact design, portability and robustness, and particularly because both the media and the writing devices are becoming less expensive. Notwithstanding the relatively high data densities offered by optical disks, still higher densities are perceived as desirable. However, in order to increase densities beyond the present values it is necessary to reduce the size of the optical beam that writes and reads the data. This has proven to be impractical without also drastically reducing the writing and reading beam strength and thus making data storage impossible. Furthermore, such optical disks typically exhibit a significant drawback in the sense that the read rates (that is the rate at which data can be read from the optical disk) are relatively low.
The present invention redresses both problems by providing an optical read/write head which permits reading and/or writing of subwavelength-scale features on optical disks at extremely high power throughputs despite the high resolution, thereby allowing for linear data densities (and therefore reading and writing rates) far higher than those allowed by the diffraction limit. (When using lenses or other far-field focusing devices to focus a light beam, the size of the convergent light xe2x80x9cspotxe2x80x9d at the focus is limited by diffraction to a diameter xcex/2 (where xcex is the optical wavelength), a phenomenon known as the diffraction limit.) Smaller spots result in higher data storage density on the optical storage medium, which in turn results in higher data read rates for a given rotational velocity of the medium. Multichannel reading and/or writing through a linear array of such read/write heads further increases the data transfer rates. Furthermore, these advantages are achieved without resorting to lasers with smaller wavelengths than are currently commercially available, so the invention offers practical application with off-the-shelf laser equipment.
Rewriteable CD-ROM optical disks commercially in use at the present time store their data as xe2x80x9cpitsxe2x80x9d on tracks on a phase-change medium. The tracks have a pitch of 1.6 xcexcm; the pit length varies between 0.4 xcexcm-1.2 xcexcm, the lower limit being given by the diffraction limit of the lasers currently in use (xcx9c780 nm for CD-ROM) since both writing and read-out occurs in the far-field, using fairly large lenses for the focusing and collection optics. Higher data densities are achieved by stacking multiple layers of storage media. Currently available DVD disks contain up to about 8 times the data of a standard CD-ROM (which contains 0.65 GB). Even though it is desirable to obtain even higher data densities, a more acute problem is the rates at which the data is read, which is currently limited by the mechanical stability of the disk rotation, and thus by the rotation speed of the disk.
To alleviate these problems, it is desirable to reduce the pit length of such optical data storage media significantly. If far-field optics are used (wherein the distance between the read/write head and the optical storage medium is much larger than the wavelength of light), the minimum pit length is given by the diffraction limit. For instance, switching to a blue-green laser would allow pit lengths of about 300 nm.
However, if near-field optics can be used (wherein a read/write head with a subwavelength-sized aperture is scanned at a height of about a few tens of nanometers above the optical storage medium) to read and/or write the data to the optical data storage medium, then the pit length (which is then limited only by the size of the read/write aperture) can be 50 nm or smaller, resulting in higher data densities as well as significantly higher writing and/or reading rates. These advances are achieved even if conventional red or even infra-red diode lasers are used, which can be made cheaply, reliably and in mass quantities. An added advantage is that the direct coupling into optical fibers or semiconductor waveguides for a near-field read/write head precludes the use of bulky and heavy collector lenses which may simplify the mechanical design of the flyhead or contact head. With such small apertures, however, the transmission through a conventional near-field device, such as a tapered optical fiber tip, suffers from severe attenuation, the result of which is a signal-to-noise ratio which is too low for reading and a lack of the high intensities necessary for writing. See E. Betzig et al., xe2x80x9cNear-Field Optics: Microscopy, Spectroscopy, and Surface Modification Beyond the Diffraction Limit,xe2x80x9d Science, Vol. 257, pp. 189-194 (1992); G. A. Valaskovic et al., xe2x80x9cParameter Control, Characterization, and Optimization in the Fabrication of Optical Fiber Near-Field Probes,xe2x80x9d Applied Optics, Vol. 34, No. 7, pp. 1215-1227 (1995). As a result, practical optical data storage read/write heads using near-field optics have not heretofore been available.
Accordingly, there is a need for a read/write head for optical data storage media using near-field optics which provides for reduced pit length size and therefore high data density and high read/write rates, and which does not suffer from severe attenuation and therefore allows both writing and reading of data on the optical storage medium, such as a phase-change medium.
Generally speaking, in accordance with the invention, a read/write head for an optical storage medium is provided. The read/write head comprises a waveguide having an end face and a plasmon-enhanced device provided on the end face of the waveguide. The plasmon-enhanced device comprises a metal film having a first surface and a second surface, the first surface being fixed to the waveguide end face, the metal film having an aperture provided therethrough. The metal film has a periodic surface topography provided on at least one of the first and second surfaces of the metal film. Light incident on one of the surfaces of the metal film interacts with a surface plasmon mode on at least one of the surfaces of the metal film thereby enhancing transmission of light through the aperture in the metal film which is directed onto and/or collected from the optical storage medium. A read/write head with an integral light source is also provided.
In addition, an array of precisely aligned read/write heads for an optical storage medium is also provided. The array comprises a plurality of waveguides, each waveguide having an end face, and all end faces being positioned substantially in the same plane, and a plasmon-enhanced device provided on the end face of each waveguide. Each plasmon-enhanced device comprises a metal film having a first surface and a second surface, the first surface being fixed to the corresponding waveguide end face, the metal film having an aperture provided therethrough. The metal film has a periodic surface topography provided on at least one of the first and second surfaces of the metal film. Light incident on one of the surfaces of the metal film interacts with a surface plasmon mode on at least one of the surfaces of the metal film thereby enhancing transmission of light through the aperture in the metal film which is directed onto and/or collected from the optical storage medium. An array of precisely aligned read/write heads with integral light sources is also provided.
Furthermore, inventive side-emitting lasers and vertical cavity surface emitting lasers (VCSELs) with enhanced transmission are also provided.
Thus, read/write heads with high resolution for optical storage media are provided. The read/write heads typically utilize near-field optics. In particular, the read/write heads are provided with a subwavelength aperture which permits the pit length on the optical storage medium to be reduced, thereby providing high data density and higher data throughput at a selected scanning speed than prior art optical read/write heads. Importantly, the optical transmission through the sub wavelength aperture in the inventive read/write head is enhanced by an interaction with surface plasmons by use of a plasmon-enhanced device (hereinafter xe2x80x9cPEDxe2x80x9d). The end face of the waveguide of the read/write head is covered with a metal film through which a sub-wavelength aperture transmits light, either in emission mode or in collection mode (or both). The enhanced transmission is the result of a resonant interaction of the light used to read or write the optical storage media and surface plasmons on the surface of the metal film of the read/write head. The resonance can be tuned to the desired wavelength by the design of surface topology of the metal film. The resolution of the device is determined by the diameter of the aperture. The transmission through the read/write head of the present invention can exceed unity (when normalized to the power incident on the area of the read/write aperture), even when the diameter of the read/write aperture is significantly smaller than the optical wavelength. See Ebbesen et al., supra; T. Thio et al., xe2x80x9cSurface-Plasmon Enhanced Transmission Through Hole Arrays in Cr Films,xe2x80x9d Journal of the Optical Society of America B, Vol. 16, No. 10, pp. 1743-1748 (1999).
Accordingly, an object of the present invention is to provide a read/write head for optical storage media which provides enhanced light transmission.
Another object of the present invention is to provide a read/write head for optical storage media which uses near-field optics.
A further object of the present invention is to provide a read/write head for optical storage media which reduces the pit length size on the optical storage media and therefore provides high data density and high read/write rates.
A still further object of the present invention is to provide a read/write head for optical storage media which does not suffer from severe attenuation and therefore allows both writing and reading of data on the optical storage medium.
A still further object of the present invention is to provide an array of read/write heads for optical storage media which are precisely aligned and provide enhanced light transmission.
Other objects of the present invention will become more readily apparent in light of the following description in conjunction with the accompanying drawings.